Compressors having an impeller rotatable within a compressor casing are well known in the art. Such compressors include both centrifugal compressors or radial flow compressors and axial flow compressors. In centrifugal or radial flow compressors, the fluid being compressed is caused by the rotating impeller to flow along a passageway in which the cross sectional area normal to the flow gradually decreases in the direction of flow. Axial compressors operate by causing the fluid to be compressed to flow along a passage of constant or substantially constant cross sectional area. An example of such a compressor is disclosed in U.S. Pat. No. 4,428,715.
Compressors of the aforementioned types may be driven by a range of motors, such as internal combustion engines, and turbines. However, in many applications it is both preferable and desirable to drive centrifugal and axial flow compressors using electric motors. Typically, induction or synchronous electric motors have been employed to drive compressors. To date, a major drawback associated with the use of electric motors to drive rotating impeller compressors has been the linkage between the electric motor and the compressor impeller. A given compressor will have a specific speed of rotation of the impeller in order to achieve the compression duty required of it. At the same time, an induction electric motor will have an optimum speed of rotation, at which the torque output is at a maximum. Heretofore, in order to link the compressor with a suitable electric drive motor, it has been necessary to employ an arrangement of one or more gears. In this way the different optimum speeds of rotation of the compressor and the electric motor can be accommodated. A particular problem arises in the case of high speed centrifugal compressors, having power requirements of the order of 200 horsepower or less. Such compressors are often required to operate at speeds in excess of 50,000 rpm. The optimum speed of rotation of an induction electric motor suitable for this duty is far lower than the speed of rotation required of the high speed compressor, requiring a gear assembly to be employed in the drive assembly of the compressor. However, for such compressors, the high costs of incorporating an arrangement of gears in the drive assembly results in a significant economical disadvantage. This in turn has led to other forms of compressors, such as screw compressors, being favored for such duties.
Accordingly, there is a need for a compressor assembly in which the requirement for a gear assembly in the drive is dispensed with and in which the compressor and the electric motor are directly linked.